Some catalysts for purifying the exhaust gas of an internal combustion engine absorb NOx in exhaust gas when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. When the oxygen concentration of the exhaust gas falls, i.e. when the air-fuel ratio changes over to stoichiometric or rich, the NOx absorbed on the catalyst is desorbed, and the desorbed NOx is reduced by HC and CO which are present in the exhaust gas (JPA 6-336916 published in 1994).
However, engine fuel or lubricating oil generally contains sulfur, and SOx (sulfur oxides) in the exhaust gas tends to be absorbed by or deposit on the catalyst more easily when the vehicle is being driven for long periods of time at a lean air-fuel ratio. If a large amount of SOx is deposited on the catalyst, absorption of NOx declines and exhaust gas purification performance is considerably impaired.
The SOx deposited on the catalyst is discharged from the catalyst when the catalyst temperature rises above its usual level. Hence, when the amount of SOx deposited on the catalyst increases, the catalyst temperature is increased to remove SOx.
In JPA 10-54274 (published in 1998), when the amount of SOx deposited on the catalyst increases and absorption of NOx by the catalyst declines, lean misfire of the engine is performed for a predetermined time. Due to this, the amount of unburnt components increases in the exhaust gas, the temperature of the catalyst increases when these unburnt components are burnt in the catalyst, and SOx is discharged.
Alternatively, the ignition timing of the engine is retarded to increase the temperature of the exhaust gas and discharge SOx from the catalyst.
However, although the temperature increase effect is large when a lean misfire is performed and unburnt components are burnt in the catalyst, it is difficult to correctly control the amount of unburnt components sent to the catalyst, and there is a possibility of the temperature rising too much and impairing the durability of the catalyst.
On the other hand, since the temperature increase due to the retardation angle of the ignition timing raises the catalyst temperature indirectly by raising the exhaust gas temperature, a sufficient temperature increase for discharging SOx is not necessarily achieved even if durability is maintained.
A method is known of improving exhaust gas composition immediately after engine startup by installing a three-way catalyst on the upstream side, i.e. in the exhaust manifold, where the temperature after startup increases quickly. However, if an NOx absorption catalyst is also installed downstream of the upstream three-way catalyst and it is attempted to raise the temperature in order to discharge SOx from the downstream catalyst, the temperature of the upstream catalyst increases too much, and these severe temperature conditions lead to early deterioration of the upstream catalyst.